i want to know why the application point to flash, what is the scenario？
why does not OS point to RAM
It is just a showcase, a real OS system would be much more complicated than this. You could also argue that why OS kernel does not have RAM in page table for this diagram.
An OS executing "in place" from e.g. NOR flash would have indeed entries in the page table pointing directly to flash.
And I agree that it would also have RAM entries.
thanks
It's too much complicated to be understood by a non IT guy like me.